The invention is directed to a corona-shielding band for electrical machines, particularly high-voltage machines.
An electrical machine is essentially composed of a stator, which is constructed of what is referred to as the plate packet wherein insulated windings are inserted into prefabricated channels, as well as of a rotor that rotates in the stator. The sheet metal packet is composed of a specific plurality of individual sheets stacked on top of one another into which the channels are punched. The stator winding is inserted into these channels, and the winding is coated with a specific insulating system dependent on the demands. A typical insulating system for high-voltage machines comprises a principal insulation that is composed of mica bands that are wound around the conductor. In the channel region, the stator winding is provided with an outside corona protection (AGS), which has good electrical conductivities, in order to prevent partial discharges in this region. The outside corona protection is thereby conducted out beyond the plate packet, so that no discharges can occur given the slight spaces relative to the pressure plates and pressure fingers of the plate packet. The windings are also impregnated with an impregnation resin with an impregnation process (VPI process) which resin is then hardened.
The electrical conditions at the channel exit of high-voltage machines correspond to those of a sliding arrangement. One electrode, i.e. the outside corona protection grounded via the plate packet, ends shortly after the plate packet; and the other electrode, i.e. the conductor, in contrast, continues farther. This arrangement yields an increase of the field strength at the insulation surface at the end of the outside corona protection. The tangential field strength ET arising therefrom along the surface of the principal insulation leads to corona and sliding discharges when the dielectric strength of the air is exceeded, and these discharges potentially destroy the insulation. In practice, this is the case beginning with operating voltages  greater than 5 kV.
Without measures for field control, the potential of the individual surface elements of the principal insulation relative to the end of the outside corona protection is defined by the two capacitances C1 (capacitance between the individual surface elements and the outside corona protection) and C0 (capacitance of the insulation between the surface elements and the conductor) and the applied voltage. For the initial voltage Ua at which the sliding discharges begin, the following derives:       U    a    ≈            E      d        ⁢                            C          1                          C          0                    
whereby Ed is the dielectric field strength of the surrounding medium. With air as ambient medium, C1 becomes small compared to C0 and the majority part of the voltage lies between the outside corona protection and the surface elements of the insulation. Given high-voltage machines having an operating voltage  greater than 5 kV, a coat, which has a defined, low conductivity, is therefore applied following the outside corona protection, and the surface capacitance C1 is shorted by this coat. The charging current for the capacitance C0 then generates a voltage drop as a result whereof the adjacent potential is gradually dismantled. This coat on the principal insulation is referred to as final corona protection (EGS) or potential control.
To this end, the insulations are usually provided with an electrically conductive layer having an adapted, specific electrical impedance. In practice, this ensues with lacquers or insulating bands to which silicon carbide is added as a conductive filler. This filler exhibits semiconducting properties in the doped condition and a highly voltage-dependent conductivity, and this means that the surface resistance of the corona protection layer decreases with increasing field strength and vice versa. This leads to a high conductivity of the final corona protection at the transition to the outside corona protection and to a low conductivity of the final corona protection at the transition to the insulation surface. A steady dismantling of the field strength up to the insulation surface is thus achieved. In practice, corona protection layers have proven themselves whose surface resistance, which is measured according to DIN IEC 167, lies between 800 and 5000 Mxcexa9 given 5 kV DC voltage.
Insulating bands, what are referred to as corona shielding bands, are usually employed for manufacturing the corona protection layer. These are composed of a carrier material composed of glass fabric or organic fabric material that is saturated with an epoxy resin (as a binding agent) that contains silicon carbide as an inorganic filler having an adaptive grain size and concentration. Silicon carbide (SiC) is employed as a conductive filler in corona shielding layers because, differing from metallic fillers, it allows the setting of the required, extremely low conductivities in the super-percolated range, and the conductivity therefore does not change significantly given processing-induced, slightly fluctuating filler concentrations. The silicon carbide is usually employed in a doped form in order to set the conductivity, particularly the voltage dependency of the conductivity, to the desired level. In practice, silicon carbide set p-conductively with aluminum has thereby proven itself. Commercially obtainable glycidylethers are employed as epoxy resins. The resins can contain an aminic hardening agent as well as a curing accelerator. Such corona shielding bands are known, for example, from the following publications: German Published Application 30 45 462, German Published Application 42 18 928 and U.S. Pat. No. 3,066,180.
The corona shielding band is wound overlapping around the outside corona protection in the region of the winding that closest to the iron core. Subsequently, the entire winding is then subjected to a VPI process (vacuum pressure impregnation) with an impregnation resin. This means that the corona shielding band that is employed must be compatible with this complex process. Thus, the band dare not contain any constituents that disturb the impregnation process or, respectively, give any such constituents off into the impregnation bath. Moreover, it must be uniformly integrated in the formed material arising after the curing so that partial discharges are avoided.
In technical employment, however, commercially obtainable corona shielding tapes which have silicon carbide as a conductive filler exhibit serious disadvantages. Thus, corona shielding tapes of the same type exhibit a great scatter in dielectric behavior (resistance level) which is dependent on the batch. This can probably be already found in the initial test of the conductivity of test members composed of a pure final corona protection; on the other hand, however, this can only be partly found at the completely insulated and VPI-impregnated windings as a result of corona discharges that occur. The technical manufacturing process of the silicon carbide is suspected as the cause. This is manufactured in a rotary tubular kiln in a reducing atmosphere from silicon carbide and carbon (Acheson process). Oxidic layers having different configurations form on the surface of the SiC particles that are formed, and these oxidic layers have a great influence on the conductivity. Dependent on the quality of the corona shielding, an unacceptable deterioration of the insulation due to increased dielectric. losses frequently results.
The conductive fillers such as lamp black, aluminum powder and silver powder that are usually technically utilized in plastics cannot be used in the present instance since they exhibit too high a specific conductivity.
An object of the invention is to make corona shielding bands available that can be manufactured in reproducible quality and that effect an optimally slight rise in the dielectric losses in the winding insulations of electrical machines.
This is inventively achieved by corona shielding bands that can be obtained in the following way.
a fabric-like carrier material is impregnated or, respectively, saturated with a solution of a reaction resin, whereby the solution also contains an inorganic filler that comprises a coating of antimony-doped tin oxide and also potentially contains a hardening agent and/or an accelerator;
following the impregnation (saturation), the solvent is removed as a result of thermal treatment, i.e. at elevated temperature.
Not only is the solvent removed as a result of the thermal treatment, but a pre-reaction also results so that the corona shielding band or, respectively, the reaction resin has hardly any stickiness and can therefore be handled in technical processes. Although a hardening does not yet occur, a xe2x80x9cpre-reactedxe2x80x9d reaction resin can already be present after the thermal treatment, i.e. a reaction resin in what is referred to as the B-condition. In this condition, the reaction resin can also be swelled or, respectively, dissolved by the impregnation resin in the later VPI process and can form a stable union therewith upon curing.
The temperature at which the thermal treatment ensues is dependent on the respectively utilized solvent, i.e. on the boiling point thereof, and on the reactivity of the reaction resin employed. Given, for example, employment of methylethylketone (boiling point 80xc2x0 C.) that is preferably utilized as solvent, the thermal treatment ensues at a maximum temperature of approximately 110xc2x0 C. For example, acetone, ethyl acetate, ethanol and toluol are suitable as solvents.
A critical feature of the invention is the specific filler of the corona shielding bands. This filler, which must be electrically conductive, is an inorganic material that comprises a coating of antimony-doped tin oxide (also referred to below as antimony-tin oxide); a type of mixed oxide is thereby present here. The proportion of the antimony in the mixed oxide advantageously amounts to 0.1 through 30% by weight. A filler having adequate semiconducting properties thus derives. Fundamentally, a pure tin oxide can also be utilized, whereby the electrical conductivity is established by a stoichiometric oxygen deficiency.
The semiconducting properties of antimony-doped tin oxide are known and are described, for example, in conjunction with the employment thereof in high-voltage insulators (see J. Indian Inst. Sci., Vol. 62 (A), March 1980, pages 83 through 88) and in conductive thermal plastic compounds (see Kunststoffe, Vol. 86 (1996) 1, pages 73 through 78). Due to the high density of approximately 7 g/cm3, corresponding fillers, however, are difficult to process. Moreover, there is the risk of inhomogeneities due to sedimentation, and this precludes their use in corona shielding layers for electrical machines.
Inorganic or, respectively, mineral fillers that are coated with antimony-doped tin oxide are known in and of themselves. They are utilized in thermal plastics and lacquers for EMS applications (electro-magnetical shielding), for example, in anti-static coats for electrical devices (see Farbe und Lack, Vol. 96 (1990) 6, pages 412 through 415 and EOS/ESD Symposium Proceedings, September 1990, pages 224 through 230).
Extensive investigations have shown that fillers coated with antimony-doped tin oxide are suitable as conductive additives in corona shielding bands for electrical machines and that, thus, the conductivity in corona shielding layers can be set very precisely and reproducibly according to the demands for high-voltage machines. Winding insulations for high-voltage machines can thus be manufactured whose dielectrical losses lie in the scope of the limits prescribed by DIN VDE 0530 Part 1. This, however, was extremely surprising and was therefore not predictable.
The filler content of the corona shielding bands of the invention generally amounts to 2.5 through 75% by weight, preferably 2.5 through 25% by weight, namely referred to the part of reaction resin as well as, potentially, hardening agent and/or accelerator.
Glass fabric and fabric of inorganic material preferably serve as carrier material, particularly in the form of fibers of polyesters or aromides, i.e. polyamides of aromatic diamines and aromatic dicarbonic acids. Insofar as they meet the demands made of insulating material for electrical machines, however, other organic fabric types can also be utilized, for example on the basis of polypropylene or fluoridated polymers. In order to keep the application onto the electrical winding as low as possible, fabric types having a GSM substance of 30 through 200 g/m2 are usually employed.
All standard reaction resins such as alkyd resins, epoxy resins, iamide resins, polyester resins and silicone resins fundamentally come into consideration as binding agent, i.e. as the resin. As a result of the balanced property profile in view of dielectric properties, temperature stability and processing behavior as well as the good compatability with the insulating system, however, epoxy resins have proven themselves to be especially suitable. Aromatic di-glycidyl ethers and polyglycidyl ethers have thereby particularly proven themselves as the insulating materials in electrical machines due to the thermal resistance.
Insofar as a hardening agent is required at all, aminic compounds are particularly utilized as the hardening agent. These are preferably aromatic amines, particularly aromatic di-amines and borotrifluoride-amine adducts, for example with mono-ethyl amine and piperidine. In particular, imidazole derivatives can serve as the accelerators. Further accelerators that can be utilized are, for example, tertiary amine and ammonia compounds.
A certain flexibility of the corona shielding bands is advantageous for a problem-free processing because they can then be wound onto the insulating surface without forming folds and pockets. A slight self-stickiness is also advantageous so that work can be carried out without additional fixing with adhesive tapes.
The fillers coated with antimony-doped tin oxide are manufactured, for example, in such a way that hydraulizable antimony and tin compounds are introduced into an aqueous filler dispersion, and the coated filler is then dried and heated (in this respect, see U.S. Pat. No. 4,373,013). Another possibility comprises coating the filler with organic antimony and tin compounds that are subsequently thermally decomposed. Oxide layers having a thickness in the range from a few nanometers to about a few hundred micrometers can be produced with both methods. By way of example, let a layer thickness of approximately 30 nm be cited.
All organic materials that are usually utilized can be employed as the filler, such as Al2O3, SiO2, TiO2, BaSO4, chalk, talcum, Kaolin, mica and titanates. Preferably, the filler is mica or a titanate such as potassium titanate, particularly in the form of whiskers.
The conductivity level of the coated fillers can be set on the basis of the content of antimony in the mixed oxide, on the basis of the layer thickness of the mixed oxide and on the basis of the grain size and the shape of the fillers. Usually, the antimony part in the mixed oxide amounts to up to 30% by weight; the layer thickness of the mixed oxide usually lies between 1 nm and a few xcexcm.
The manufacturer of the corona shielding bands ensues according to the standard methods for the manufacture of insulating bands for electrical machines. Solutions of the reaction resins are thereby utilized wherein the semiconducting filler is dispersed (the mixtures of reaction resin, solvent and filler ready for processing as well as, potentially, hardening agents and/or accelerators are also referred to below as reaction resin compounds). The viscosity and, thus, the application onto the fabric materials is defined by the concentration of the reaction resin and of the filler in the solution. The fabric materials can, as bands having a greater or lesser width, either be drawn through the solution or sprayed with the solution. The band then passes through a horizontal or vertical drying path at elevated temperature in order to remove the solvent. Subsequently, the band is wound up.
The invention shall be explained in yet greater detail on the basis of exemplary embodiments (MT=mass parts).
For manufacturing a reaction resin compound, the reaction resin is dissolved in a solvent at room temperature. The calculated quantities of hardening agent or, respectively, accelerator are then potentially added to the solution and dissolved while stirring. Subsequently, the calculated part of an electrically conductive filler is uniformly distributed in the resin solution with the assistance of a dissolver. The constituents employed in the examples are compiled in Table 1.
For manufacturing corona shielding bands, a fabric band as the carrier material is drawn through a container filled with the reaction resin compound at a defined speed and is thereby impregnated. The resin supply is continuously agitated before and during the trial implementation in order to prevent a depositing of the conductive filler. After the impregnation, the fabric band is conducted through a drying tower with four hot zones that can be regulated independently of one another. Work was carried out given the following drying conditions (MEK as solvent): 60xc2x0 C., 90xc2x0 C., 110xc2x0 C. and 70xc2x0 C.; band velocity: 20 cm/min.
The corona shielding bands manufactured in the described way are respectively wrapped single-ply with a 50% overlapping onto a reaction glass of Duranglass (according to DIN 12395) having an outside diameter of 30 mm. Subsequently, the samples are hardened at 160xc2x0 C. in the pre-heated ambient air kiln. The electrodes needed for the electrical contacting are annularly painted on with highly conductive, air-drying silver conductive paint at a 10 mm spacing according to DIN IEC 167, VDE 0303 Part 61 Section 7; the power terminals are implemented with bare copper wire. The resistance measurement ensues according to DIN IEC 167, VDE 0303 Part 31 with a highly constant DC voltage source in the range from 1 through 6 kV in 1 kV steps.